The present invention relates to mass flow measuring systems and, more particularly, to mass flow sensor housings that substantially eliminate thermal gradients that might otherwise be imposed upon a mass flow sensor.
Capillary tube thermal mass flow sensors exploit the fact that heat transfer to a fluid flowing in a laminar tube from the tube walls is a function of mass flow rate of the fluid, the difference between the fluid temperature and the wall temperature, and the specific heat of the fluid. Mass flow controllers employ a variety of mass flow sensor configurations. For example, one type of construction involves a stainless steel flow tube with two or more resistive elements in thermally conductive contact with the sensor tube. The resistive elements are typically composed of a material having a high temperature coefficient of resistance. Each of the elements can act as a heater, a detector, or both. One or more of the elements is energized with electrical current to supply heat to the fluid stream through the tube. If the heaters are supplied with constant current, the rate of fluid mass flow through the tube can be derived from temperature differences in the elements. Fluid mass flow rates can also be derived by varying the current through the heaters to maintain a constant temperature profile.
Such thermal mass flow sensors may be attached to a mass flow controller, with fluid from the controller""s bypass tube feeding the capillary tube (also referred to herein as the sensor tube). Since mass flow measurements are dependent upon the temperature effects of the fluid upon the resistive elements, any external differential temperature imparted to the resistive elements could produce errors in the measurement of a mass flow rate. Unfortunately, thermal mass flow sensors are frequently operated in environments where an external thermal gradient might be imposed upon them. For example, a thermal mass flow sensor may be operated in close proximity to a valve coil that dissipates significant power as it operates. Heat generated from operation of the valve coil may be communicated through a conductive thermal path, such as that provided by a mass flow controller housing, to the mass flow sensor. The heat thus-communicated may impose a thermal gradient upon the mass flow sensor housing which could, in turn, superimpose the external thermal gradient upon the sensor""s resistive elements, thus leading to errors in mass flow measurements.
A mass flow sensor that substantially eliminates externally imposed thermal gradients would therefore be highly desirable.
In a mass flow sensor in accordance with the principles of the present invention a mass flow sensor housing is attached to a mass flow controller through a baseplate. The mass flow sensor includes a mass flow sensor tube oriented along a predetermined axis within the mass flow sensor housing. The baseplate may be integral to the sensor housing or it may be attached to the housing through any of a variety of attachment means, such as threaded through-holes and bolts, for example. The baseplate is configured to provide a thermal path between the mass flow controller and the sensor to thereby maintain the sensor and controller housings at substantially the same average temperature. Additionally, the thermal path provided by the baseplate is configured to substantially reduce or eliminate thermal gradients which might otherwise be imposed upon the mass flow sensor housing through thermally conductive contact with the mass flow controller housing.
In an illustrative embodiment a sensor housing establishes a thermal ground between the mass flow sensor housing and the mass flow controller housing. The thermal ground features a cross-sectional area that is significantly less than the cross sectional footprint of the sensor housing. The cross-section of the thermal ground may have a circular, rectangular or other geometrical shape. The thermal ground provides a thermal path that is sufficient to maintain the overall average temperatures of the mass flow sensor housing and the mass flow controller housing at substantially the same level. Additionally, the thermal ground is located in an area of the sensor housing substantially coincident with the midpoint of the flow sensor tube. The thermal path created by the thermal ground is sufficient to maintain the average temperatures of the sensor housing and mass flow controller housing substantially equal.
The permissible lag time between a change in the average temperature of the mass flow controller housing and a corresponding change in the average temperature of the mass flow sensor housing is a design choice that will affect the accuracy of mass flow readings. Given a permissible lag time (for given a temperature shift), a corresponding thermal flow figure may be computed. The thermal flow figure may then be used to determine the thermal conductivity and cross sectional area required for the thermal ground material. To minimize the possibility of the establishment of thermal gradients across the flow sensor housing, the cross sectional area of the thermal ground is minimized. That is, for a convenient structural material, such as Aluminum, the cross sectional area of the thermal ground must be large enough to eliminate temperature differentials between the thermal mass flow sensor housing and the thermal mass flow controller housing, yet small enough to prevent the establishment of thermal gradients across the cross-section of the thermal ground.
In an illustrative embodiment, a mass flow sensor housing includes thermal ground having a rectangular cross section. The thermal ground is oriented orthogonal to the axis of the flow sensor tube, and is positioned substantially midway between the sensor tube input and output apertures. A mass flow sensor housing in accordance with the principles of the present invention is particularly well suited for use with a mass flow sensor such as a differential current thermal mass flow transducer. The thermal ground may be formed in a way that permits relatively easy adjustment of the thermal ground""s position relative to the mass flow sensor. In such an embodiment the position of the mass flow sensor may be adjusted to xe2x80x9czero outxe2x80x9d the effect of a mass flow sensor""s thermal clamp.
Additionally, a mass flow controller in accordance with the principles of the present invention includes a sensor assembly, a valve assembly, and a mass flow controller housing to which the valve and sensor assemblies are attached. A thermally conductive element conducts heat from the valve assembly away from the sensor assembly, thereby reducing uncontrolled contributions of heat to the sensor assembly. In an illustrative embodiment, the mass flow controller includes a thermally conductive enclosure that substantially envelops the sensor and valve assemblies. The thermally conductive element substantially surrounds and makes thermally conductive contact with the valve assembly while, at the same time, making substantial conductive thermal contact with the enclosure.
In an embodiment that does not include the enclosure, the thermally conductive element may include structure on one or more surfaces that accelerate the dissipation of thermal energy from within the valve assembly. Such structure may include fins located on one ore more exterior surfaces of the thermally conductive element that do not face the sensor assembly. The thermally conductive element may be composed of a high thermal conductivity material, such as aluminum. In an illustrative embodiment, the thermally conductive element is integral to the valve assembly. That is, in this illustrative embodiment, the exterior wall of the valve assembly is formed to conduct thermal energy away from the mass flow controller sensor assembly.
These and other advantages of the present disclosure will become more apparent to those of ordinary skill in the art after having read the following detailed descriptions of the preferred embodiments, which are illustrated in the attached drawing figures. For convenience of illustration, elements within the Figures may not be drawn to scale.